The great increase in the number of automotive vehicles has created an air pollution problem throughout the world. As a result, much effort has been expended in an attempt to decrease the amount, not only of hydrocarbons and carbon monoxide in the exhaust gases, but also of nitrogen oxide, because of the formation of smog by the photochemical reaction in the air and its irritation of the eyes and respiratory systems of human beings.
The production of hydrocarbons and carbon monoxide occurs because of insufficient combustion in the engine cylinders. It is possible to decrease the amounts of those components considerably, by carrying out the combustion completely, as by providing a sufficient amount of air at high temperatures. Improvement in combustion efficiency results in an increase in the thermal effect of the engine. An increase in combustion temperatures, however, increases the production of nitrogen oxide. In contrast thereto, a lowering of the maximum combustion temperature has a tendency to suppress an increase in the combustion efficiency of the engine and to prevent, as a result thereof, the formation of nitrogen oxide in the exhaust gas.
Various measures have been proposed in an attempt to find a solution to the foregoing problem, such as controlling the spark advance, providing a mixture of gas that avoids the peak air-fuel ratio, recycling a part of the exhaust gas, injecting water and using a catalytic converter, but none of those measures has been entirely practical or satisfactory, and they often present new problems.
In the case of changing the spark advance, the output power tends to decrease and the fuel cost tends to increase in proportion to the reduction in the rate of nitrogen oxide production, and, in addition, the value of the decrease in nitrogen oxide cannot be made as large as is desired because the angle through which a change can be effected is limited. In the case of changing the mixing ratio of air-fuel, when the gas mixture is relatively thin or lean, an ignition failure frequently takes place, causing an instability in the number of revolutions of the engine, whereas, when the gas mixture is relatively rich, the contents of hydrocarbons and carbon monoxide in the exhaust gases becomes large and the fuel cost increases.
In the case of recycling the exhaust gas, in which a part of the exhaust gas is subjected to reintroduction into the engine by the suction effect of the latter, the ignition becomes unstable because of the presence of the uncombusted substances, and carbon or tar-like substances tend to be deposited in the system, such as in the suction pipe and the suction vane. In the water injection system, it is necessary to use as much water as fuel, and the apparatus for controlling the amount of water increases the cost of the vehicle. A satisfactory catalytic system with the desired purifying ability, the required service life and the desired operating costs has not yet been developed.